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10 key points covered in our CFD Meshing Tips & Tricks webinar
Aug15

10 key points covered in our CFD Meshing Tips & Tricks webinar

A quick thanks to the large number of customers in Australia and New Zealand who attended our July webinar on ANSYS CFD Meshing Tips & Tricks.  We've had many enquiries from people wanting to know more, so we thought we'd break the content down into the 10 key points below.  If you want more information on any specific point, then please contact us directly or post a comment in the field at the bottom of the page.    10 key points to a successful CFD Meshing strategy (taken from the demonstration throughout LEAP's webinar on CFD Meshing Tips & Tricks): Firstly, decide what mesh connectivity your problem requires (conformal / non-conformal) and how this will affect the setup of your geometry - single part, multi-body parts or separate bodies.  Utilise the tools available for geometry clean-up - using DesignModeler or SpaceClaim Direct Modeler - to quickly address any small surfaces, split edges, hard edges that are present in your CAD. Decide whether you can use a patch-conforming meshing approach (preferred for most CFD cases), or need to use a patch-independent approach to tackle dirty CAD geometry. Make use of the preview tools for previewing the surface mesh and inflation layers.  This can be a great time saver for large, complex models! Use the show tool to indicate if there are bodies that are automatically sweepable, and/or faces that can be easily map meshed. Make sure you are using the correct meshing preference - talk to us if you need more information on different meshing requirements for Mechanical, CFD, Explicit Dynamics problems. Understand when it is helpful to use Assembly meshing.  If Assembly meshing is part of your strategy, remember that Fluent should be selected as Solver type which gives you access to the Cut-cell and Cut-tet methods.  CFX users can use this same approach to generate Cut-tet meshes. If you are hex meshing, it is useful to understand how to use Sweep meshing controls most efficiently. The ability to display nodes and edge parametric directions are very handy! Don't forget you have the ability to use Virtual Topology and Pinch commands to cleanup geometry in ANSYS Meshing. If you are dealing with large assemblies and/or non-conformal meshes, the automatic contact detection is a tool you cannot live without. Use it to check connectivity of bodies with the new Body View tool. Don't forget the importance of 'group by none' for CFX users.  If you weren't able to attend the webinar, or did but simply want more information on what was covered, please let us know below or contact LEAP's technical support hotline.  For more information on upcoming events, please visit LEAP's webinar, training and events...

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Tips & Tricks: Convergence and Mesh Independence Study
Jan17

Tips & Tricks: Convergence and Mesh Independence Study

The previous posts have discussed the meshing requirements that we need to pay attention to for a valid result. It is important to remember that your solution is the numerical solution to the problem that you posed by defining your mesh and boundary conditions. The more accurate your mesh and boundary conditions, the more accurate your "converged" solution will be.   CONVERGENCE Convergence is something that all CFD Engineers talk about, but we must remember that the way we generally define convergence (by looking at Residual values) is only a small part of ensuring that we have a valid solution. For a Steady State simulation we need to ensure that the solution satisfies the following three conditions:   - Residual RMS Error values have reduced to an acceptable value (typically 10-4 or 10-5) - Monitor points for our values of interest have reached a steady solution - The domain has imbalances of less than 1%.   RMS Residual Error Values   Our values of interest are essentially the main outputs from our simulation, so pressure drop, forces, mass flow etc. We need to make sure that these have converged to a steady value otherwise if we let the simulation run for an additional 50 iterations then you would have a different result. Ensuring that these values have reached a steady solution means that you are basing your decisions on a single repeatable value.   Example of Monitoring a Value of Interest   As a rule, we must ensure that prior to starting a simulation we clearly define what our values of interest are, and we make sure that we monitor these to ensure that they reach a steady state. As previously highlighted, we also need to make sure that the Residual RMS Error values are to at least 10-4. Finally, we need to ensure that the overall imbalance in the domain is less than 1% for all variables.   Imbalances in the Domain     MESH INDEPENDENCE STUDY The approach outlined above results in a single solution for the given mesh that we have used. Although we are happy that this has "converged" based on RMS Error values, monitor points and imbalances, we need to make sure that the solution is also independent of the mesh resolution. Not checking this is a common cause of erroneous results in CFD, and this process should at least be carried out once for each type of problem that you deal with so that the next time a similar problem arises, you can apply the same mesh sizing. In this way you will have more confidence in your results.   The way we...

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Tips & Tricks: Inflation Layer Meshing in ANSYS
Jan06

Tips & Tricks: Inflation Layer Meshing in ANSYS

Throughout our first set of Tips & Tricks posts relating to meshing controls and meshing methods, we have made mention of Inflation Layers a number of times.  Let's take this opportunity to explain exactly why inflation layers are a critical component of a good CFD mesh and how we can create them easily within ANSYS Meshing. In our first posts on Mesh Sizing we explained that as well as capturing all key features of the geometry (using local sizing and the curvature size function), we also need to have a sufficiently fine mesh to adequately capture regions where the flow will experience rapid change in key variables such as pressure, velocity or temperature.   This initially led us to a better understanding of how we should apply Global and Local Mesh Controls. Now, if you think about moving a probe from the freestream flow towards one of the walls in your fluid domain, as you approach the wall you will notice that the velocity decreases non-linearly up to a point where the fluid will have zero velocity at the wall.  This is what is termed the "no slip" wall condition in CFD. If we plot a typical velocity profile in the near-wall region, we can see that we have a large change in velocity in the wall normal direction and it is important to our CFD simulation that we capture this gradient correctly.  To do this, we need to use inflation layer meshing to accurately capture the boundary layer region for any wall-bounded turbulent flows.  The image below plots the non-dimensional velocity versus the non-dimensional wall normal distance, with each line from top to bottom demonstrating the difference between a favourable pressure gradient through to adverse pressure gradient with flow separation.  It is clear that the flow behaviour in the near wall region is fairly complex and needs to be captured appropriately to have any confidence in our CFD results, especially if we intend to report key engineering data such as separation points or pressure drops.                     Providing a suitable inflation mesh for the geometry is strongly tied to the choice of the turbulence model, and the flow field we are interested in capturing. We can elect to resolve the complete profile of the boundary layer of alternatively we can make use of empirical wall functions to reduce the cell count (see our post on turbulence modelling and wall functions). If we refer to the images below, on the left hand side we observe that the boundary layer profile is modelled with a reduced cell count, which is characteristic of a wall function approach. On the right, the boundary layer profile is resolved all the way to the wall....

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How the top Formula One (F1) teams stay on the podium
Dec05

How the top Formula One (F1) teams stay on the podium

You may already know that Computational Fluid Dynamics (CFD) is currently used by all Formula One teams as part of their ongoing development for their aerodynamics and cooling packages. Since the technologies pioneered in F1 car development regularly flow through into other engineering industries, we felt that a post on this topic was the perfect way to launch LEAP's CFD blog.   In recent years, CFD has become a tool that is no longer just a complement to F1 team's sophisticated wind tunnel and track testing equipment, but an indispensable tool for the top teams to gain that extra competitive advantage. In this post, we will focus on why and how this powerful technology is used by Formula One teams today, as well as global automotive manufacturers who employ similar techniques (within slightly longer timeframes).   Formula One is a unique sport from a technical perspective, where all teams manufacture and design components from a blank sheet to conform to a certain set of rules. These rules are interpreted differently by all teams and as such there is considerable design freedom (amid regular rule changes and updates). Ultimately, this design freedom has a significant impact on the performance of F1 car designs during on-track testing, qualifying and during real race conditions.   The aerodynamic design process must be extremely efficient for a team to remain competitive, as any inefficient period will inevitably lead to a loss of performance compared to other teams that have continued along an efficient development path. This is why supercomputers and wind tunnels run 24/7 at most Formula One team headquarters.   Typical Formula One Super Computer Courtesy of SAUBER PETRONAS Engineering AG   Most teams will adhere to the following development process.   Initial CFD analysis of multiple concepts Further CFD development of the most promising concepts Wind tunnel tests of developed concepts Track confirmation of new parts (usually on the Friday before a race)   This whole process can be accelerated if the performance improvements are found to be significant enough, and potentially any idea (even relatively radical changes) could be designed, tested, and raced within just a two week period.   For an aerodynamicist, the use of CFD is a vital tool in Formula One, as it provides repeatable results in a controlled environment that is sometimes not achievable even in the controlled environment of a $200 million wind tunnel. The aerodynamicist has the freedom to design without limitations on mounting points, prototyping lag times or mechanical issues that may otherwise prevent physical testing. The increasing use of CFD has led to a process that encourages creative design and rapid development of...

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